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Afterglow and the Violet Wire
As some of you are probably aware, if you snip/remove the violet wire from the GP relay, you can get your GP's to glow for an extended period after the engine is running. This should theoretically reduce smoke, emissions, and coking issues. The problem is that snipping the wire also forces the GP's to run 'afterglow' even when the engine is warm, which will diminish the life expectancy of the plugs. Many folks have wired in a manual switch to turn off this feature, but I wanted a more 'hands-free' operation.

I installed a thermally switched relay into the GP relay's violet wire circuit. I used an NC (normally closed) relay from a Volvo 700 series fusebox. These are a 5 terminal relay, similar in look to the familiar 4 terminal NO (normally open) versions used in Euro headlight conversions and countless other little projects. These NC relays have an additional 87a terminal to which one side of the violet wire is connected, opposite the 30 common for the other side. Be careful of other similar 5 terminal relays with an 87b terminal--these are just NO relays with two bridged terminals--these won't work.

I piggy-backed onto the 50 C thermal switch above the thermostat housing on my '87 300TD OM 603, which is apparently designed to assist and smooth cold shifting in the transmission.

When the relay is energized, the violet wire is snipped--afterglow enabled. In this phase, the 50 C thermo-switch has continuity with ground.

When the relay is not energized, the violet wire is bridged--afterglow is disabled. In this phase the thermo-switch has no continuity with ground.

I did this about five years ago and have been extremely pleased with the reduction in smoke and initial roughness on those cold morning startups.


The 86 terminal is wired to the temp switch on the t-stat housing.

The 85 terminal is wired to a 12v keyed source (w/inline fuse).

The 87a terminal is wired to one side of the snipped violet wire.

The 30 terminal is wired to the other side of the snipped violet wire.

The 87 terminal is not used.

The violet wire is snipped approximately 2" back from the relay, which requires trimming the harness cover a bit.

Factory solution
(write-up and pics borrowed from Dave M., gsxr)

There are two choices for factory afterglow relays, either the 3.5L relay with up to 60 seconds of afterglow max, or the later OM606 relay with up to 180 seconds max. The factory setup uses a coolant temp sensor in the cylinder head and determines the length of afterglow on coolant temp. The chart (time vs temp) is in the factory OM603 engine manual, either paper or CD-ROM, if you want to check it out. The manuals have no such chart for the the 606 relay however. Above about 40C there is nearly zero afterglow.

For the 1986-87 OM603 engine with vertical injection, you need 25mm long glow plugs. Don't use plugs from the newer 603 engines, they are shorter (23mm) and although they may work OK they're not optimal. You can either get Bosch or Beru. The easiest to get are Bosch Duraterm. The hardest to get are Bosch Duraterm Chromium which must come from Germany. Beru falls in the middle. Expect to pay $11-$13 per plug at a minimum. I wouldn't put in new plugs other than Bosch or Beru. You can do the relay upgrade with your existing plugs but don't be surprised if they don't last long.

Other than the relay, you need the coolant temp sensor, which replaces the sensor in the cylinder head for the dash gauge. It's about $30-$35 wholesale. You also need two pin bushing housings (black connector widgets) and two small metal pin bushings, a few feet of wire (16ga works fine), and finally a soldering iron to solder the wire to the bushings. The whole setup works great. When cold the plugs pull an amazing 120A total, after a few seconds it reduces to 80-90A, and after the engine starts the current drops as low as 48A total. Definitely a good idea for folks who live in cold climates and/or have to park outside.

Note the vast difference between relay internals. The stock relay appears to connect all 6 plugs together with a brass plate, the afterglow relay may not... neat stuff.

INSTALLATION NOTE: The two prongs of the new 2-prong temp sensor are NOT identical. One is for the dash gauge, the other goes to the glow relay, but the resistance measurements are vastly different. I forget which pin is which, but if you measure the old 1-prong dash gauge sensor, match that reading with one of the two pins on the new one.


Premium Member
5,450 Posts
Good stuff. Too bad I couldn't snag that low miles 87 sedan a few months ago. Woulda put a big smile on my face.
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